Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream electronically so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is particularly well suited for application of characters onto irregularly shaped surfaces, including, for example, the bottom of beverage containers.
In general, an ink jet composition must meet certain rigid requirements to be useful in ink jet printing operations. These relate to viscosity, resistivity, solubility, compatibility of components and wettability of the substrate. Further, the ink must be quick-drying and smear resistant, must be capable of passing through the ink jet nozzle without clogging, and must permit rapid cleanup of the machine components with minimum effort.
However, despite the many advantages of ink jet printing, it has not, to date, been used to print images on postage stamps. To date, an ink composition which both meets the requirements for use in ink jet printing and the requirements for printing on postage stamps has not been found.
The printing of images on postage stamps, such as cancellation marks, is currently accomplished by contact printing, which involves the use of printing rollers. The cancellation marks printed in this manner dry slowly, and often do not pass the stringent tests for indelibility conducted by postal authorities throughout the world. Because print rollers are used, information appearing on the cancellation mark, such as the date, must be changed manually every day. This is obviously time-consuming. Further, contact printing allows only a limited amount of information to be printed.
Although ink jet printing would appear to present a useful means for printing stamp cancellation marks, ink jet formulations to date, have not been uniformly suitable for printing on all stamp varieties. Obviously, it is an absolute requirement for any cancellation method that the cancellation mark be uniformly and consistently printed on all stamps. Accordingly, ink jet printing, to date, has not been suitable for printing cancellation marks.
Further, the ink must meet the indelibility tests conducted by the various post authorities for stamp cancellation ink. To meet these tests, the ink must be at least resistant to water, and aqueous solutions of acetone, methanol, ethanol, glycol ethers, and detergents. Finally, in addition to meeting the special requirements for stamp cancellation, the ink must meet the general requirements listed above relating to viscosity, resistivity, etc., before it can be used in an ink jet printing operation.
Heretofore, an ink jet composition meeting all of the above-noted needs has not been available. Such needs are now satisfied by the present invention, the description of which follows.